Electrodeless discharge lamps have many advantages over the conventional incandescent lamps, including higher efficiency, lower power consumption and longer life. Even though the discharge lamps cost more to manufacture initially, the extra initial cost is more than offset by the above advantages, resulting in a lower overall operating cost over time.
Although traditional fluorescent discharge lamps utilizing electrodes are now in common use and do have similar advantages to the electrodeless lamps, they cannot be manufactured in the same compact package. As a result, they cannot replace incandescent lamps where a fairly compact efficient unit is required. Alternatively, tungsten halogen lamps are available in compact units and are also more efficient than conventional incandescent lamps. However, they are less efficient than discharge lamps, may emit unwanted ultraviolet radiation, operate at higher temperatures and are also more vulnerable to mechanical shock.
Typically, electrodeless discharge lamps have a discharge vessel which is sealed in a gas tight manner and filled with a gaseous mixture comprising a metal vapor and a rare gas. An alternating current is fed through an induction coil in close proximity with the discharge vessel, generating an electromagnetic field within the discharge vessel. This electromagnetic field excites the gaseous mixture inside the discharge vessel, producing electromagnetic radiation by cycling between energy states. The electromagnetic radiation is then converted into visible light by a fluorescent layer on a surface of the discharge vessel.
However, a serious side effect of feeding an alternating current into an induction coil is the production of electromagnetic radiation which is transmitted into regions beyond the discharge vessel. In addition, the high frequency electronic circuitry also adds to the production of unwanted electromagnetic radiation. Unfortunately, some of the frequencies that can be generated by these electrodeless lamps fall within the wide band of radio frequencies reserved by the FCC for wireless communication equipment. For example, although the FCC has allocated 13.56 MHz for ISM (industrial, scientific and medical) uses, including the lamps, the fifth harmonic of 13.56 MHz falls within the range 64-71 MHz reserved for television broadcast of Channel 4. As a result, these discharge lamps can interfere with the operation of wireless communication equipment unless measures are taken to reduce the intensity of the electromagnetic radiation.
Various methods of reducing this unwanted radio frequency radiation have been proposed but they all have significant disadvantages. One method involves the vapor deposition of a transparent layer of electrically conductive material, typically tin oxide, on the inner surface of the discharge vessel. This layer is then grounded to the power supply. Such a method is disclosed in U.S. Pat. No. 4,728,867, where a fluorine-doped layer of transparent tin oxide is deposited onto the inner surface of the discharge vessel. This layer is then grounded by means of a metal spring.
Several variations of this evaporated transparent tin oxide layer have also been suggested. Some disclose use of the transparent conductive layer over selected areas of the inner surface of the discharge vessel. Others disclose the inner transparent conductive layer in combination with other forms of shielding.
U.S. Pat. No. 4,940,923 discloses the initial vapor deposition of a wide horizontal strip of transparent, electrically conductive aluminum (thickness approximately 2 microns) onto the inner surface of the discharge vessel. From this aluminum strip, three rings are formed by removing parts of the strip using a laser beam from the outside. This layer is then grounded by a wire connected to the inner conductive layer by penetrating the wall of the discharge vessel.
Yet another variation of an inner transparent conductive layer is disclosed in U.S. Pat. Nos. 4,568,859 and 4,645,967, where a transparent conductive layer of tin doped indium oxide is deposited on the inner surface of the discharge vessel. In addition, three copper rings are disposed around the outside surface of the discharge vessel at the level of the induction coil in grooves provided for this purpose.
However all the above-described methods which disclose the vapor deposition of a transparent conductive layer on the inner surface of the discharge vessel suffer from several serious problems. First, the conductive layer, being disposed within the discharge vessel, is exposed to the hostile metal vapor environment. Second, some means of conduction must be provided to ground the inner conductive layer to the exterior of the discharge vessel. Third, any conductive layer or strip that provides an unbroken circular electrical path may behave like a "shorted turn" to the induction coil and further reduces the efficiency of the discharge lamp. Fourth, this inner layer vapor deposition process must also be well controlled so as to deposit just the right thickness of the conductive material to satisfy the trade off between electrical conductivity and light transmission.
Other, non-shielding methods have also been suggested. Examples include U.S. Pat. No. 4,171,503 which discloses an induction coil wound in the form of a toroidal helix to minimize the microwave radiation leaks. Such a shaped coil is employed to minimize microwave radiation associated with cylindrical induction coils. However, toroidal helix shapes are costly to manufacture.
U.S. Pat. No. 4,254,363 discloses several windings comprising transparent tin oxide stripes deposited over selected portions of the discharge vessel. However these windings carry current and function as the source of the magnetic field rather then as an electromagnetic radiation shield.